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For thousands of years mankind has used waterpower on a small scale, for example, in mills for grinding corn, and such use continues in many parts of the world today although it has largely ceased in the UK. Hydropower, the term generally used for the generation of electricity from waterpower, was developed in the late nineteenth century. The first hydro-electric power station is thought to have been built by the industrialist William Armstrong at Cragside, Northumberland, UK, in 1870.

Power can be generated where water falls through a height, known as the ‘head’ (measured vertically). The power, P, generated in kW, is calculated as follows:

As a source of renewable energy, hydropower has considerable appeal, especially when part of a multi-purpose scheme. However, capital costs tend to be several times that of the cheapest thermal powerplant. Its multi-disciplinary nature (e.g hydrology, hydraulics, geology; and geotechnical, civil, structural, electrical and mechanical engineering), means that hydropower poses some of the greatest challenges of any engineeringprojects. Large schemes can also have significant sociological and environmental impacts which may be hard to predict. Because of their potential impacts, large schemes are often controversial. All aspects, even for small schemes, warrant careful feasibility studies.

The installed capacity of hydropower worldwide is listed by the International Hydropower Association to have been 1,036 GW in 2014 (IHA, 2015). World annual hydropowerelectricity production is stated by the International EnergyAgency to have been 3,756 TWh in 2012, 2.3% of total primary energysupply (IEA, 2014).

Comparable figures are published by the World EnergyCouncil (WEC). WEC reports a resurgence in development since 2004 following a near cessation in 1999 on commencement of a review of the effectiveness of large dams by the World Commission on Dams (WEC, 2015). At the end of 2014 the estimated share of global electricity production was 16.6% (REN21, 2015).

The proportion of a country's total electricity production generated by hydropower depends on its geographical characteristics and the extent to which its potential has been developed. For example, Paraguay generates almost 100% of its energy from hydropower but a dozen or more low-lying countries generate less than 1% of their energy from hydropower. Combining hydropower with flood control, irrigation, water supply, navigation and recreation can make such schemes economically feasible.

In the UK, the total installed capacity at the end of 2011 was 1.676 GW, about 1.9% of the total UK generating capacity, most of which is in large schemes in Scotland (DECC, 2013). The energy produced in an average year is about 5.7 TWh which is about 1.5% of the UK's total electricity production.

By comparison, large thermal power stations in the UK are typically 2.0 GW and produce in the order of 10 TWh per year of energy. Most of the hydropower capacity was built in the 1950s and 1960s, absorbing much of the larger scale opportunities in the UK.

Incentives for developing renewable resources have contributed to a resurgence in interest in new projects and in refurbishing old schemes, even some old watermills. The figures quoted above (world and UK) are for ‘primary’ power only. They do not include the pumped storage schemes, such as Dinorwig and Ffestiniog in North Wales with capacities of 1,728 MW and 360 MW respectively, and Foyers (300 MW) and Cruachan (400 MW) in Scotland.

Land in the west and north of the UK is at a comparatively high elevation and receives plenty of rain, but the remainder is generally low lying and has less rainfall. This is the reason that the majority of the country's hydropower is generated in the north of Scotland.

Even there, average elevation is low and catchment areas small by comparison with continental mountainous regions, so the potential for the economic development of hydropower is relatively small. By comparison, Norway and Switzerland generate 95% and 56% of their electricity from hydropower respectively, and the installed capacities of the largest schemes in China and Paraguay are measured in thousands of megawatts.

The UK does, however, have significant untapped potential tidal and wave energy resources if the technological, economic and environmental challenges can be overcome.

Hydropower schemes can be classified as; run of river, storage or impounding or pumped storage schemes. Tides can also be used to generate power, either by forming a barrage across a bay or estuary with a substantial tidal range or by using the kinetic energy of the flow where the velocity is high.

Sizes of schemes vary considerably. The smallest run of river schemes (of which there are many in ruralcommunities around the world or that are privately owned) range from just a few kilowatts. At the other end of the scale, the Three Gorges Dam in China has an installed capacity of 22,500 MW.

There are also several types of plant, in two main generic groups, reaction and impulse, that can be used to convert the head (potential energy) of the water into electrical energy. The most widely used machines are Francis turbines (reaction type) which operate over a wide range of head and can have efficiencies in excess of 90%.

Pelton wheels (impulse type) tend to be used for the highest head schemes. Several other types of machine are used, many of which may be cheaper to manufacture but are less efficient. For example, some low head schemes use Archimedean screws. Impounding tidal projects may be equipped with Francis turbines; kinetic tidal schemes can have a propeller type or vertical or horizontal turbine, or reciprocating hydrofoils.

Run-of-river schemes abstract the flow, or a proportion of it, available in the river at an intake, conveying it by canal or tunnel (known as the headrace) at a gentle gradient until a useable head (from just a few metres to some hundreds of metres) is available above the river. To be economic, the gradient of the river may be of the order of 5% while that of the headrace may be of the order of 1/1000 or less. A pipeline or shaft is used to deliver the flow to the power station.

Run-of-river schemes have the advantage that they do not change the flows in the river except over the length of the scheme. Conversely, their output varies and is restricted by the flow available at any time. The efficiency of turbines falls as the flow reduces below the design capacity and generation stops completely at low flows; the economics can be very sensitive to variations in rainfall and intermittency is an issue for plants not connected to a grid.

In a storage or impounding scheme, the head is created or augmented by the construction of a dam. Water in the reservoir created by the dam can be released to match diurnal or seasonal variations in demand, still limited of course by the total inflow. Energy output may be restricted in dry years through lack of inflow and because, as the level of water in the reservoir is drawn down, the head and thus the output of electricity is reduced.

Pumped storage schemes take advantage of surplus electricity at off peak times from 'base load' stations (particularly nuclear) or increasingly from renewable sources such as wind turbines. Most use Francis type turbines which perform the dual role as pumps with only a small loss of efficiency to pump water from a lower reservoir to an upper reservoir.

In generation mode, the water is discharged through the turbines back to the lower reservoir. The water is used repeatedly requiring a relatively small in-feed to make up for evaporation and any leakages.

Even though capital costs tend to be high and the overall efficiency of a scheme may be not much more than 70%, pumped storage schemes remain (in 2015) the dominant way of storing substantial amounts of energy in a form that is readily convertible to electricity (though other technologies such as compressed air energy storage and batteries are beginning to offer alternatives).

They perform a valuable function in balancing supply and demand, providing a reserve of generating capacity to respond to changes in load or sudden failure of other generating plant or transmission links, and for frequency control. Especially if held in ‘spinning reserve’, that is spinning in air and synchronised to the grid, turbines can be brought on load within a few seconds.

Pumped storage schemes ideally require large upper and lower reservoirs separated by as much vertical but little horizontal distance as possible. Very few sites provide these attributes naturally; typically the upper reservoir is substantially artificial with little or no natural catchment, tends to be small and is therefore subject to significant change in water level.

The lower reservoir is usually larger and the changes in level are therefore less. Occasionally the lower 'reservoir' may be a large river or the sea. The lower reservoir may be isolated from the wider environment to prevent harmful influences. For example the river which previously flowed into Llyn Peris, which was adapted to form the lower reservoir for the Dinorwig scheme, is now diverted around it in a tunnel to avoid its natural flow being affected by the scheme.

Tidal power is still very much in the developmental stage with the exception of a few projects including La Rance (240 MW) near St Malo in France built in the early 1960s, the Bay of Fundy in Canada (20 MW, 1984) and the Sihwa Lake tidal powerplant in Korea (254 MW, 2011), which are all barrage schemes. Tidal barrage schemes are potentially economic in the relatively few places worldwide where tidal ranges are the highest.

For example, spring tide ranges are 14.5 m and 13.5 m at the Bay of Fundy and La Rance respectively. A barrage across the River Severn estuary, where the spring tide range at about 11 m is also exceptionally large, has been studied and proposed for many years; it could provide up to about 5% of the UK's energy but remains highly contentious, both environmentally and economically. A 320 MW tidal lagoon in Swansea Bay was given Government approval in June 2015.

Kinetic or dynamic generation schemes operate where tidal stream velocities are highest which tends to be where there are constrictions between land masses such as in estuaries (e.g. The Humber) or between islands (e.g. around the Orkney Islands). The highest tidal velocities are often in the same regions as the highest tidal ranges, because they are generated by the general progression of the tides around the Earth being impeded by land masses, but are not precisely coincident. Kinetic tidal generation technology is still very much in the developmental stage with a limited number of prototype or demonstration schemes in operation around the world.

They have the advantage of minimal environmental impact, though potentially presenting a hazard to shipping, but of course they operate necessarily in the most hostile marine environments so are expensive and difficult to install and maintain, and it is difficult to achieve adequate reliability. In early 2015, construction of Phase 1a (6 MW) of the 398 MW MeyGen project started in the Pentland Firth off the north coast of Scotland.

Another related technology, though not strictly hydropower in the way defined at the beginning of this briefing, is wave energy. This technology is also at the developmental stage with a variety of devices being tested on coastlines exposed to ocean waves with the highest energy intensity.

Tidal and wave power is estimated to be able to meet up to 20% of the UK's electricity needs (DECC, 2013). At present there is no identifiable feasible and economic path to the development of such a major contribution though some development is likely as technology improves and the drive for renewable sources increases.

The World EnergyCouncilestimates that the global undeveloped hydropower potential is about 10,000 TWh annually (WEC, 2015). About 424 GW was under construction in 2011 (WEC, 2013). A larger amount is under planning but, whilst the potential remains significant, environmental concerns and funding difficulties make the timings of future developments uncertain.

The pressure to reduce carbon emissions, rising energyprices and incentives such as feed in tariffs make continuing development of hydropower attractive. Small run-of river schemes can be relatively easy to develop; larger impounding schemes are more likely to be developed where there is sufficient capability to meet base load demand, such as in Canada, or where benefits from irrigation and water supply as well as hydropower combine to make them economically attractive, such as may be the case in hotter climates. Pumped storage schemes are attractive where there is a high premium for peak loadelectricity, as in the UK.

The largest new scheme under construction in 2014, Cia Aig in Scotland, will have a capacity of 3 MW. The majority of new schemes planned and identified in the UK are smaller but a pumped storage scheme of 600 MW is planned at Coire Glas with an alternative similarly sized scheme at Balmacaan, both also in Scotland.

The technically exploitable capability of hydropowerplants in the UK is about 14 TWh annually. The development of more large schemes is unlikely because the best sites have already been developed. Because of the UK's topography, hydropower cannot be a major contributor to the national requirement for primary energy but there is a remaining viable potential of 850-1,550 MW in small scale resources (DECC, 2013). Existing channels leading to abandoned mills, or weirs which exist to allow navigation on rivers, provide some opportunities for small schemes in lower lying areas.

Possible exceptions to these limitations include:

Pumped storage schemes (such as noted above) because of their value in balancing supply and demand; intermittency of generation will become more significant as large base load generation stations close and more variable sources of power such as wind, solar and tides come online.

Tidal energy (such as the Severn, Swansea and MayGen schemes referred to above) which has a potential of between 25 and 30GW (DECC, 2013).

Hydroelectric powerprojects, particularly impounding schemes, have a wide range of potential impacts. Projects built before the present emphasis on environmental and social impacts might not be promoted nowadays or might have been designed better to mitigate harmful effects. Impacts can include displacement of people, destruction of habitat and fertile land, unsightly mud banks at low reservoir water level, siltation (reducing effective storage volume), and loss of water through evaporation from the large surface area of the reservoir.

In the UK, siltation is less significant where vegetation limits sediment loads, and evaporation losses are not high. Siltation is important in 'young' mountain ranges with high erosion rates; evaporation is important in hot climates.

If vegetation is not removed before inundation, there can be a significant release of methane, a much more potent greenhouse gas than carbon dioxide. There can be rapid changes in the flow in the river downstream when generation starts or ceases unless the reservoir discharges directly into another reservoir, or to the sea. Dams are a barrier to the migration of fish. For very large schemes there may be unpredictable micro-climate changes.

At least some of the disadvantages can be mitigated. Limiting drawdown of a reservoir still allows generation of much of the available energy as the majority of the volume of water is in the upper range of the reservoir where the area of the water surface is greater and the head is the maximum.

"Compensationwater" is released to prevent flows in rivers drying up completely and "freshets" of increased water flow are sometimes released to simulate a more natural flow variation in the river. Fish passes are built to allow migratory fish to pass dams and reach their breeding grounds in the headwaters upstream of the reservoirs.

On the positive side, many reservoirs have acquired a considerable amenity value, in some instances being designated SSSIs because of the habitat provided for wildfowl, while others provide flood protection for communities downstream. Many are used for recreational facilities and for the development of tourism.

The reputation of schemes perceived as damaging may have enhanced opposition to new projects, possibly inhibiting reasonable progress in developing countries seeking to improve the health and standard of living of their populations. In the UK there is a similar risk that the extent to which environmental impacts can be addressed will not be recognised, and that schemes which would have a beneficial impact overall meet considerable opposition.

Hydropower can and should be an important part of strategies for reducing carbon emissions. However, for the reasons outlined above, all hydropowerprojects require thorough study and planning in order to evaluate their technical and economic feasibility, and environmental and social impacts.

For example, careful hydrological, geotechnical, environmental and other studies are necessary; the size (installed capacity) of the scheme needs to be optimised and the likely energy production determined.

Costs, including debt, and financial benefits must be analysed carefully. Sensitivities to inaccuracies in all assumptions, especially forecast rainfall and flow, and costs, must be tested. It has been suggested that large hydropower dams will be too costly in absolute terms and take too long to build to deliver a positive risk-adjusted return (Ansar et al, 2014) though this has been contested by ICOLD (ICOLD, 2014) and others. Thorough risk assessment and management is essential, addressing the wide range of variables to which projects may be subject.